This invention relates to liquid level detecting or sensing systems and, more particularly, to a pulse heated thermal detector wherein the effective thermal capacity of a thermistor sensor is used in conjunction with induced pulsed resistance excursions to determine the liquid level.
Liquid level detecting systems are known in the art and have been used for detecting the relative or absolute level of a liquid in a vessel such as, for example, the tank of a hot water boiler system. In these applications, it is particularly desirable to sense a dangerously low liquid level situation so as to preclude damage to the boiler tank. A widely used prior art technique has been to project an electrode probe into the boiler tank and in electrical series with an alarm circuit wherein the liquid itself is used as an electrical conductor to complete an electrical circuit when the water is above the level of the probe. These systems are not only expensive to install, but are also subject to contamination and fouling of the probe in the hostile environment of the tank.
More recently, liquid level detecting systems have employed probes which include a highly thermally conductive member such as a metallic pin one end of which projects into the tank for contact with the liquid therein. The other end of the pin is in contact with a heater such as a resistance heater. The other end of the pin is also in contact with a precious-metal contact assembly wherein the thermal expansion coefficient of the contact assembly is used in conjunction with the difference in thermal conductivity at the projecting end of the probe as the liquid level varies to provide a closure to activate an external alarm. However, these devices require temperature compensation for changes in ambient temperatures; and by definition the device includes moving parts which are subject to failure.
Prior art liquid level detecting systems have also utilized thermistor devices as an integral part of a probe projected into a liquid containing vessel, and wherein the resistance variation of the thermistor as a function of temperature is utilized to provide a liquid level indication. That is, the absence or presence of the liquid varies the total or absolute resistance of the thermistor. Here again, however, these devices require temperature compensation such as the use of a second thermistor at a remote location which is used to cancel variations in ambient temperature.
These and other disadvantages are overcome by the present invention wherein there is provided a pulse heated thermal liquid level detector having no moving parts and wherein ambient temperature variations do not affect the operation of the sensor.